fn main() {
    let mut x = 5;

    println!("the value of x is {}", x);

    x = 6; // cannot assign twice to immutable variable -> let mut 变成可变
    println!("the value of x is {}", x);

    const SECONDS: i8 = 60; // cannot be mutable
    println!("the value of SECONDS is {}", SECONDS);
}
